Wavelength selective filter

ABSTRACT

A tunable optical wavelength selective filter is constituted by a dynamic holographic diffraction element (3) in combination with a fixed diffraction grating or hologram (2). The dynamic diffraction element (3) is preferably implemented as an electronically controlled image displayed on a pixelated spatial light modulator and in particular a spatial light modulating using photo-electronic integrated circuits fabricated using silicon VLSI technology and integrated with ferro-electric liquid crystals. Amongst other uses the filter can be implemented to form a digitally tunable laser.

TECHNICAL FIELD

The present invention relates to a wavelength selective filter and atunable laser including it.

BACKGROUND ART

Many different types of optical filters for selecting light of aparticular wavelength have been proposed in conjunction with wavelengthdivision multiplexed optical systems for use in optical communicationsystems.

Also the use of a single fixed diffraction grating for passive opticalwavelength separation is well understood. The spectral components of theinput source are separated and distributed around an output plane sothat selection of the correct spatial region allows any spectralcomponent to be isolated. Thus, spatial filtering of the output planeproduces spectral filtering of the source. Such a technique can be usedto demultiplex a wavelength division multiplexed signal consisting of anumber of data-modulated wavelength channels provided that the size ofthe output devices are correctly matched to the data bandwidths.

A single fixed diffraction grating only separates the differentwavelength channels and allows no recombination of these channels in theoutput plane. Re-selection of the output wavelengths i.e. somehow tuningsuch a filter can only be achieved by changing the grating, rotating itor somehow moving the spatial filtering device in the output plane. Inthe past these have been implemented by some form of servo-mechanicalsystem with a feedback control to cause physical movement of the gratingor of a photodetector or optical fibre receiving light in the outputplane.

An example of an optical crossbar switch using a variable hologram toroute light in a particular direction between its input and output andso, in effect, cause switching between a particular input of thecrossbar switch and a particular output is described in an articleentitled "A Holographicly Routed Optical Crossbar: Theory andSimulation" by D. C. O'Brien, W. A. Crossland and R. J. Mears publishedin Optical Computing and Processing, 1991, volume 1, number 3, pages233-243.

DISCLOSURE OF INVENTION

According to a first aspect of this invention a tunable opticalwavelength selective filter comprises a dynamic holographic diffractionelement in combination with a fixed diffraction grating or hologram.

Such a filter can be used in coherent and semi-coherent spread-spectraapplications and is likely to be used principally within the visible andinfra-red frequency band.

The filter in accordance with this invention relies upon the fixeddiffraction grating or hologram to disperse light of differentwavelength into its constituent spectral components or, for example,wavelength-multiplexed data streams, and then using the additionaldynamic holographic diffraction pattern for tuning the filter so that byaltering the holographic diffraction pattern present on the dynamichologram it is possible to alter the wavelength or wavelengths whichleave the particular angle of interest. The combination of a dynamicholographic diffraction element and a fixed holographic element may alsobe arranged arbitrarily to recombine various of the wavelengths in theoutput plane.

The dynamic diffraction element preferably is some type ofopto-electronic interface which can take a number of forms dependingupon the actual application. The dynamic diffraction element ispreferably implemented as an electronically controlled image displayedon a pixellated spatial light modulator and in particular a spatiallight modulator using opto-electronic integrated circuits fabricatedusing silicon VLSI technology and integrated with ferro-electric,nematic or electroclinic liquid crystals. Such spatial light modulatorsare well suited to telecommunication applications in terms of speed,reliability and ease of interfacing to control electronics. Such devicesare readily controllable, typically via a computer to display one of aseries of different holographic diffraction patterns. Typically suchholograms are 2-dimensional optical phase and/or amplitude gratingswhich produce a controllable deviation and dispersion of the incidentlight but which can also be arranged to control optical fan-out andfan-in operations, to generate multiple output beams by splitting theinput beams into two or more such output beams, and to deflect the inputbeams in two dimensions rather than one.

The fixed grating may simply have the form of a fixed regular amplitudegrating or phase plate. When the fixed grating has the form of a phaseplate it may be an etched glass plate and may be physically combinedwith the dynamic holographic diffraction element by, for example, beingetched into its outer surface. The fixed hologram may be computergenerated.

For both holographic components phase-mode operation rather thanamplitude-mode operation provides greater diffraction efficiency andtherefore lowers signal loss through the filter. In addition,multi-level or blazed holograms or gratings can be used to maximise theefficiency by preferentially diffracting the light into the desiredangular orders.

Preferably, the filter further comprises one or more multi-wavelengthinput sources and one or more output channels. The input source may takethe form of an active multi-wavelength laser or semi-coherent devicewhich is possibly spatially filtered or be the end of a cleaved fibre orwaveguide. Multiple inputs can be accommodated either by using a singlecollimating lens so that the spatial positioning of the inputs isconverted into an initial angular multiplexing through the system whichis super-imposed on the output positions, or by separately collimatingeach of the sources using a multi-way lens array and then dividing theholograms into multiple sub-holograms so that each one is uniquelyilluminated by a different input source. In addition a combination ofboth methods could be used. The filter preferably includes an outputlens to convert the output angular direction of the light beams into amore useful spatial separation by wavelength.

The two holograms or gratings may be either transmissive or reflective.In the latter case beam splitters could be used, or the holograms couldbe tilted to separate the incident and diffractive paths. A reflectivearchitecture allows the possibility of compacting the system by foldingits optical axis back on itself so that the input and outputs then liein the same plane, and so that a single lens can be used for both theinput and output. The output channels can be detected using lightsensitive photodiodes, a charge coupled device array or can be launcheddirectly into a single array of output waveguides or fibres. When thespectral components are spatially distributed in the output plane thephysical size of the output channels determines the bandwidth orspectral range collected by each output.

The filter in accordance with the invention is optically transparent,that is, filter tuning is achieved without the need to convert theoptical input signal into an electrical signal and then back again to anoptical signal once again. Once the filter has been tuned, the inputdata rate of such a filter is only limited by the source modulationrates and not by any part of the filter.

According to a second aspect of this invention, a tunable laser includesa filter in accordance with the first aspect of this invention withinits optical cavity.

Such a tunable laser is thus electronically digitally tunable withoutmoving parts and with the geometry of the configuration permitting ahigh degree of wavelength stability and repeatability. Such a laser maybe used in test equipment for fields such as communication wherecalibration and stability are key features and may be used as a sourceof light in wavelength division multiplexed applications oncommunication networks or spectroscopy.

BRIEF DESCRIPTION OF DRAWINGS

Examples of the present invention will now be described with referenceto the accompanying drawings in which:

FIG. 1 is an optical diagram of a tunable filter;

FIGS. 2a and 2b are optical diagrams showing optical filters with twodifferent input schemes;

FIG. 3 is an optical diagram of an example of a tunable filter with afolded optical axis;

FIGS. 4 and 5 are graphs showing the performance of the example of FIG.3;

FIG. 6 is an example of a uni-directional tunable fibre ring laser;

FIG. 7 shows the output of a Fabry-Perot laser diode source;

FIG. 8 shows a digitally filtered spectrum of FIG. 7; and

FIG. 9 shows the selection of lasing wavelengths using the tunable fibrering laser of FIG. 6.

DETAILED DESCRIPTION

This section describes the design of a wavelength selective filtercomprising of:

one or more multi-wavelength input sources 1;

a fixed grating or hologram 2;

a dynamic grating or hologram 3; and

one or more output channels 4.

The principle of operation of the wavelength filter is the angularseparation and selection of wavelengths using a coarse dynamic hologram3 to tune onto a highly wavelength dispersive, fixed hologram 2(although the sequence of the dynamic hologram 3 and fixed hologram 2may be reversed). If all the spectral components of the illuminationsource 1 are parallel and collimated when they enter the filter, eachcomponent will leave the two hologram combination in a dynamicallycontrollable direction. By altering the dynamic hologram pattern, it ispossible to alter which wavelengths λ₁, λ₂, λ₃ . . . λ_(n) leave at theparticular angular directions of interest. The output angular dispersionof these collimated beams can then be converted into a more usefulspatial wavelength-separation by means of a lens 5, FIG. 1. Arbitraryspatial filtering of the output plane 6 therefore selects arbitrarywavelength components from the input source. These wavelengths have beentuned to their current output plane positions by the dynamic hologram 3.

The input source 1 could take the form of an active multi-wavelengthlaser or semi-coherent device (possibly spatially filtered), or of acleaved fibre or waveguide. Multiple inputs can be accommodated eitherusing a single collimating lens 7 so that the spatial positioning of theinputs is converted into an initial angular multiplexing through thesystem which is superimposed on the output positions, or by separatelycollimating each of the N sources using an N-way lens array 8 and thendividing the holograms into N (different) sub-holograms 9 so that eachone is uniquely illuminated by a different input source, FIGS. 2a and2b. In addition, a combination of both methods could be used.

The dynamic hologram 3 would probably be implemented as anelectronically controlled image displayed on an amplitude- or phase-modeSLM; the fixed hologram 2 simply consists of an amplitude- orphase-plate (e.g., etched glass). For both components, phase-modeoperation, rather than amplitude-mode, will provide greater diffractionefficiency and therefore lower signal loss through the filter. Inaddition, multi-level or blazed holograms can be used to maximise theefficiency by only diffracting the light only into the desired angularorders.

For some embodiments, one or both holograms may deflect the input beamsin 2-dimensions, rather than the 1-dimensional diagrammatic form shownin FIG. 1. They may also be used to generate multiple output beamdeflections, i.e., the input beams are split, so that each wavelengthcomponent appears in the output plane more than once per input. Hencearbitrary space and/or wavelength switching between multiple inputs andmultiple outputs may occur using an optical fan-out and fan-inprinciple.

The two holograms may be either transmissive or reflective or acombination of the two. In the reflective case, beam splitters could beused, or the holograms could be tilted to separate the incident anddiffracted paths. A reflective architecture additionally allows thepossibility of compacting the system by folding the optic axis back uponitself, thus combining f₁ and f₂ into a single lens. The inputs andoutputs would then lie in the same plane.

The output channels could be detected using light-sensitive photodiodes,a charge-coupled device array (CCD) or could be launched directly intoan output waveguide, waveguide array, fibre or fibre array. Because thespectral components are spatially distributed in the output plane, themode size of the output channels determines the bandwidth (i.e.,spectral range) collected by each output. If the holograms are reducedto regular gratings then from diffraction theory, the wavelengtharriving at a particular point in the output plane is approximatelypredicted by equation (1) below. If either (or both) holograms split theincident light beam in more than one direction, this equation may besolved using the set of equivalent regular grating pitches. ##EQU1##where λ is the wavelength selected by the filter, x is the inputdevice's displacement from the system's optic axis,

y is the output device's displacement from the system's optic axis,

d_(F) is the fixed holographic pitch,

d_(SLM) is the dynamic holographic pitch, (pitch of one pixel)

n is an integer in the range -N/2→+N/2 representing the tuning of thefilter,

N is the SLM dimension in number of pixels.

The dynamic hologram 3 modifies the light's incident angles onto thefixed hologram 2 in fine angular steps, hence tuning the switch. Thegreater the ratio between the fixed and dynamic holographic pitches, thefiner the wavelength tuning step produced.

Control of the filter and hence selection of the filtered spectralcomponents arriving at the output 4, simply relies on the generation anddisplay of the correct holographic images on the SLM. The set ofpossible holograms can be pre-generated and stored in electronicnon-volatile memory behind the filter and then transferred to the SLMwhen required.

The number of images required, and hence the total tuning range of thefilter, depends on the number of SLM pixels that are used. Providedd_(F) <<d_(SLM), the filter will exhibit an almost constant wavelengthtuning step over much of the filter range.

An important feature of the proposed design is that in a fibreinput/fibre output environment, it is optically transparent. Once thefilter has been tuned, the input data rate is essentially limited onlyby the source modulation rates and the filter bandwidth.

The optical tunable wavelength filter principle has now been describedand an actual embodiment is shown schematically for a singleinput/single output filter as FIG. 3.

In the scheme shown, the optic axis has been folded back upon itself byutilising a reflective phase grating as the fixed hologram 2 and using atransmissive SLM as the dynamic hologram 3. This system has beenexperimentally demonstrated. The fixed hologram used was simply aregular 1-dimensional aluminium coated grating with 50 line pairs/mm.The cross-section of this grating is a square wave with an etch depth ofλ/4≈370 nm to give π phase operation at commercial fibre opticwavelengths around 1.5 μm.

The SLM has a pixel pitch of 180 μm and has been optimised for use atsimilar infra-red wavelengths. It is to be used to tune the filter bydisplaying a pre-generated series of non-regular 1-dimensionalholograms, orientated in the same direction as the fixed grating lines.

The input and output channels are both single-mode fibre with corediameters of about 10 μm.

In one architecture the optical beam may be passed through the dynamichologram, onto the fixed grating, and then back through the same, (or anidentical) dynamic hologram. The overall result is that the two passesof the dynamic hologram exactly cancel one another out, thus removingany angular variation of approach to the output, but retaining thetuning ability of the filter. Provided the fixed hologram is placedexactly a focal distance, f, from the lens so that the system becomes a2f architecture, there will always be a single, unique path from theinput to the output and the particular wavelength (or multiplewavelengths) that follow this path are determined by the dynamicholographic image being displayed on the SLM. In addition, thischaracteristic is independent of the exact position of the SLM. Thealignment conditions of the architecture are therefore no worse than fora non-tunable WDM multiplexer.

A property of the type of the ferro-electric SLM used here is that it isonly capable of displaying binary holograms, i.e., each pixel has onlytwo states of transmittance of either +1 or e^(j)θ when it is operatingin a phase-mode, or 0 and +1 in amplitude-mode. Binary holograms alwayscontain rotational symmetry in their output fields. Thus for everycomputed diffraction order, there exists a symmetric order of equalintensity. The symmetric order modifies equation (1) and introducesextra solutions: ##EQU2## where n is now an integer ##EQU3##

It can be seen from (2) that λ_(CENTRE) is always a solution of thefilter (regardless of the dynamic tuning), but that the output spectrumwill also contain one tuned spectral component at a higher wavelengthand another at a lower wavelength than this fixed component. Bydesigning the filter such that this centre wavelength and one of thesidebands lie in non-active regions of the input spectrum, a generalisednon-symmetric tunable filter for single-mode fibre applications canstill be achieved using binary dynamic holograms.

If a different SLM capable of multi-level holograms were used instead,the above problem (caused by the double pass of the SLM) will not ariseunless we attempt to fan-out the incident beam into several diffractionorders.

A complete experimental investigation of this particular embodiment hasbeen performed using binary holograms, to verify its correct operation.The single-mode fibre-tailed input source used during the experiment hadthe spectrum shown in FIG. 4. The filter was arranged such that theupper sideband and centre filter solutions lie to the right of thisspectrum (i.e., at a higher wavelength). The lower sideband solution wasthen tuned by displaying various holograms on the SLM to isolate thethree major modes of the input spectrum. This is shown as the threeplots of FIG. 5. These plots are the measured spectra launched into thesingle-mode output fibre.

The high loss of the current experimental filter is mainly due to thevery poor performance of the SLM that was used and the need to placethis device between crossed polarisers. The majority of the remainingloss could be eliminated by using a blazed or multi-level grating as thefixed hologram.

A tunable filter allows additional switching functionality to beincorporated into the demultiplexer design. A tunable filter of the typeproposed has a wide range of applications including:

i) Analysis, filtering and/or shaping of coherent spread-spectrasources, including applications involving time-dependent measurements.

ii) The generation of a tunable laser source by placement of the filterin the resonant cavity of a laser.

iii) The implementation of a tunable single or multiple wavelengthcoherent source(s) using a broadband coherent source as input.

iv) The implementation of a tunable wavelength switch or receiver(s) forselection of one or more wavelengths through one or more outputchannels, using a multi-wavelength input source or sources.

Applications (iii) and (iv) are important components in theimplementation of multi-wavelength communication networks, especially inWDM environments, including high-density systems (HDWDM). Specialconsideration is made in this design of the need to couple light backinto single-moded optical-fibre for these applications.

The proposed design implements a tunable non-mechanical (de)multiplexeror tunable cavity laser by placement of a combination of a fixed(holographic) grating and a dynamic holographic grating in the path of acollimated light beam.

The fixed grating achieves a high degree of wavelength separation,providing good linearity over the tuning range within a compact size.

The dynamic hologram provides a means of tuning the filter inpotentially very fine wavelength increments.

The size of the system could be reduced further by etching the fixedgrating or hologram directly onto a glass face of the SLM dynamichologram device.

Alignment conditions of the design are no worse than those of a fixedWDM (de)multiplexer because the design functionality is independent ofthe exact positioning of the dynamic hologram between any initialcollimating lens and the fixed hologram.

The filter is optically transparent. That is, filter tuning is achievedwithout the need to convert the optical input signal into an electricalsignal and back to an optical one again.

The construction of a tunable fibre ring laser using the wavelengthselective filter of the present invention, is shown in FIG. 6. Lightfrom an input single-moded communications fibre 10 (φ_(c) ≈10 μm)aligned along the optical axis 11 is collimated by a doublet lens 12optimised for use in the near infra-red and of focal length f=96.1 mm.The beam is passed through the SLM 3 and diffracted by the displayedbinary phase hologram, which is effectively a one dimensional grating.The diffracted orders are then further diffracted and angularlydispersed by a fixed binary phase grating 2, which has been optimised togive a π phase change at a wavelength of 1550 nm. The light then passesthrough a similar doublet lens 13 which spatially separates theangularly separated diffracted orders. The second lens 13 is also placedat an angle of 5° to the optical axis 11, so as to omptimise thecoupling efficiency of the output fibre 14 placed a distance x=8.5 mmfrom the optical axis 11 in the focal plane of the second lens 13 and onthe optical axis of lens 13, which collects light from only onediffracted order. Thus for a particular hologram displayed on the SLM 3,only one wavelength will be coupled back into the second fibre 14.Tuning of the filter is performed simply by changing the spatialfrequency of the hologram displayed on the SLM.

The SLM 3 is a transmissive multiplexed glass cell with 128×128 pixelson a 165 μm pitch that has been optimised to act as a half wave platearound the 1.5 μm erbium window and can be reconfigured in under 5 ms.The fixed transmission grating 2 is made with a spatial period of 18 μmand is fabricated by spinning a layer of photoresist on a glass flat toa depth of ##EQU4##

(where n_(o) is the refractive index of air, n₁ is that of thephotoresist ≈1.6) and using photolithography to selectively etch abinary phase grating.

The holograms are digital binary-phase pixellated images which aregenerated using an iterative algorithm such as simulated annealing. Theholograms may be designed to optimally direct light of a fixedwavelength to a single spot anywhere in the first order, or to fan outthe light to multiple spots. This can be extended so that the hologramcan optimally direct a single desired wavelength or multiple wavelengthsof light to a fixed point in the output plane. The equation governingthe wavelength to be coupled back for the filter in FIG. 6 is givenapproximately by: ##EQU5## where λ is the wavelength, x=8.5 mm is thedistance of the output fibre from the optical axis, f=96.1 mm is thefocal length, N=128 is the number of pixels in the SLM, D=165 μm is theSLM pixel pitch, d=18 μm is the period of the fixed diffraction grating.The factor n/ND represents the equivalent spatial frequency of thedisplayed hologram, where n is an integer between 0 and N/2. Binaryphase holograms produce symmetric outputs so that half the light isalways lost. Coupled with a diffraction efficiency of 81% and the sinceenvelope, each grating contributes a loss of 4.4 dB. The FLC SLM canalso only diffract sin² 2θ of the incident light, where 2θ=28° is theswitching angle of the FLC, thus causing a loss of 6.6 dB. coupling ofonly 40% between the two fibres causes a loss of 4.0 dB whilereflections cause an additional 1.8 dB loss. With other losses, thetotal loss through the filter is currently 22.3 dB, but this could besubstantially reduced by optimisation of the optical materials andcomponents. The tuning speed of the filter is determined by the frameupdate rate of the SLM and the inherent redundancy in holograms makesthe system extremely robust in the event of individual pixel failures.

The laser source for characterisation of the filter is a Fabry Perotlaser diode source with multiple lasing modes, as shown in FIG. 7,spaced 1.2 mm apart. The filter successfully isolates the individualmodes. An arbitrary filter result is shown in FIG. 8. In this example,the FWHM of the filter is 2 nm, which is almost diffraction limited, andthe filter can be tuned in steps of 1.3 nm over a theoretical range of82 nm. Finer resolution and a smaller FWHM could be achieved byincreasing f or the fixed grating resolution, subject to the availableSLM clear aperture.

Placing the filter and an erbium-doped fibre amplifer 15 in auni-directional fibre ring resonator forms a digitally tunable fibrering laser. For monitoring a 5:95 coupler 16 placed in the amplifier 15is used at the laser output 17.

In tests, lasing was achieved over the range 1543 to 1566 nm in discretesteps with an average spacing of 1.3 nm. The measurements were recordedusing an optical spectrum analyser. FIG. 9 shows an arbitrary group ofseven successivly tuned lasing modes. The signal from the 5:95 coupler16 was attenuated by a further 23 dB to avoid possible damage to thespectrum analyser. Substantial output power could also be accessed byplacing a fibre in the zero order of the SLM and collecting theundiffracted light. This would obviate the need for an additionalcoupler in the fibre loop.

When holograms are designed, there are many possible solutions whichtune to ostensibly the same wavelength. But in fact the solutions allhave slight variations in their spatial frequencies and output phaseprofiles. This ultimately causes fine differences in the wavelengthwhich is most efficiently coupled back into the second fibre, andconsequently the lasing frequency.

What is claimed is:
 1. A tunable filter for polychromatic opticalradiation comprising an electronically programmable spatial lightmodulator for displaying computer generated hologram patterns of data asa series combination of a first dynamically variable wavelengthdispersive element, and a second static wavelength dispersive element.2. A tunable filter according to claim 1, in which the second staticwavelength dispersive element (2) comprises a fixed hologram.
 3. Atunable filter according to claim 1, in which the second staticwavelength dispersive element (2) is a phase plate.
 4. A tunable filteraccording to claim 3, in which the phase plate is physically combinedwith the first dynamically variable wavelength dispersive element (3).5. A tunable filter according to claim 1, in which the second staticwavelength dispersive element (2) is an amplitude grating.
 6. A tunablefilter according to claim 1, further comprising one or moremulti-wavelength input sources (1) and one or more output channel paths(4).
 7. A tunable filter according to claim 6, in which the one or moreinput sources are selected from the group consisting of an activemulti-wavelength laser or a semi-coherent device.
 8. A tunable filteraccording to claim 6, including a collimating lens (7) so that thespatial positioning of the one or more input sources (1) is convertedinto an initial angular multiplexing through the filter.
 9. A tunablefilter according to claim 6, including a multi-way collimating lensarray (8) for separately collimating each input source (1).
 10. Atunable filter according to claim 1, in which the first dynamicallyvariable wavelength dispersive element (3) comprises an array of Nsub-holograms (9).
 11. A tunable filter according to claim 1, in whichthe second static wavelength dispersive element (2) comprises an arrayof N fixes sub-holograms.
 12. A tunable filter according to claim 1,including an output lens (5, 7) to convert the output angular directionof incident light beams into a spatial separation by wavelength.
 13. Atunable filter according to claim 1, in which one or both of the firstdynamically variable and second static wavelength dispersive elements(2,3) are reflective.
 14. A tunable filter according to claim 13, inwhich the first dynamically variable wavelength dispersive element (3)comprises a back plane ferro-electric liquid crystal spatial lightmodulating device.
 15. A tunable filter according to claim 13, whereinsaid tunable filter is configured so as to be folded about an opticalaxis so that the input (1) and output (4) lie in the same plane.
 16. Atunable filter according to claim 1, further comprising a memory storingdata for at least one computer generated hologram which when displayedin use, causes multiplexing of wavelengths in a predetermined manner.17. A tunable laser including a tunable filter for polychromatic opticalradiation within an optical cavity of said tunable laser, said tunablefilter comprising an electronically programmable spatial light modulatorfor displaying computer generated hologram patterns of data as a seriescombination of a dynamically variable first wavelength dispersiveelement, and a second static wavelength dispersive element.
 18. Atunable fiber ring laser including a tunable filter for polychromaticoptical radiation, said tunable filter comprising an electronicallyprogrammable spatial light modulator for displaying computer generatedhologram patterns of data as a series combination of a first dynamicallyvariable wavelength dispersive element, and a second static wavelengthdispersive element.